A wide variety of implanted medical devices (IMDs) for delivering a therapy or monitoring a physiologic condition which can employ one or more elongated electrical leads and/or sensors are available. Such IMDs can monitor or deliver therapy to the heart, muscle, nerve, brain, and stomach or other organs. Examples of such IMDs include implantable cardioverter defibrillator devices, which have a pulse generator and one or more electrical leads with one or more electrodes that conduct signals to and receive signals from the patient's heart.
These electrical lead(s) and their electrode(s) are placed in or proximate to the organ such that an electrical signal between electrodes is capable of stimulating the organ. The electrodes may be configured either to deliver a stimulus to the organ, or to detect or sense an intrinsic electrical event associated with the organ. A programming device or programmer communicates with the medical device through a communication link. One example of a communication link is a telemetry link that provides means for commands and data to be non-invasively transmitted and received between the programmer and the device.
Leads associated with IMDs typically include a lead body extending between a proximal lead end and a distal lead end and incorporate one or more exposed electrode or sensor elements located at or near the distal lead end. One or more elongated electrical conductors extend through the lead body from a connector assembly provided at a proximal lead end for connection with an associated IMD to an electrode located at the distal lead end or along a section of the lead body. Each electrical conductor is typically electrically isolated from any other electrical conductors and is encased within an outer sheath that electrically insulates the lead conductors from body tissue and fluids.
Implantable medical leads can extend from a subcutaneous implantation site of the IMD through an internal body pathway to a desired tissue site. The leads are generally preferred having small diameter, highly flexible, reliable lead bodies that withstand degradation by body fluids and body movements that apply stress and strain to the lead body and the connections made to electrodes. As lead bodies are made smaller and smaller and the number of lead conductors is increased or maintained, the integrity of lead conductors is increasingly important.
Continuous flexing of the cardiac lead bodies due to the beating of the heart, for example, is also an important consideration in maintaining the lead integrity. Other stresses are applied to the lead body during an implantation or lead repositioning procedure. Movements by the patient can cause the route traversed by the lead body to be constricted or otherwise altered causing stresses on the lead body. At times, the lead bodies can be slightly damaged during surgical implantation, and the slight damage can progress in the body environment until a lead conductor fractures and/or the insulation is breached. The effects of lead body damage can progress from an intermittent manifestation to a more continuous effect. In extreme cases, insulation of one or more of the electrical conductors can be breached, causing the conductors to contact one another or body fluids resulting in a low impedance or short circuit. In other cases, a lead conductor can fracture and exhibit an intermittent or continuous open circuit resulting in an intermittent or continuous high impedance.
Such lead issues resulting in short or open circuits, for example, can be referred to, for simplicity, as “lead-related conditions.” In the case of cardiac leads, the ability to sense cardiac activity conditions accurately through a lead can be impaired by these lead-related conditions. Complete lead breakage impedes any sensing functions while lead conductor fractures or intermittent contact can demonstrate electrical noise that interferes with accurate sensing. During cardiac pacing or defibrillation, lead-related conditions can reduce the effectiveness of a pacing or defibrillation below that sufficient to pace or defibrillate the heart.
To detect these lead-related conditions, certain pacemakers and IMDs have been provided with the capability of storing cardiac electrogram data prompted by the automatic determination of oversensing or undersensing of cardiac events, loss of capture, out of range lead impedance measurements, etc. Such data is telemetered to an external instrument when the physician interrogates the IMD and used by the clinician to try to pinpoint a suspected lead-related condition. Another example for detecting a lead-related condition is based on lead impedance measurements. Such cardiac rhythm management systems have included circuitry that is capable of measuring the lead impedance (the impedance seen by the device across a pacing lead or sensing lead). Other solutions have included detecting a lead-related condition based on automatic detection of capture failure. One such method of detecting lead-related conditions has a dedicated unipolar evoked response channel with low capacitance that can detect the evoked response of the cardiac muscle activation following the pacing artifact. When the evoked response following the pacing pulse falls below an evoked response threshold, loss of capture is declared. A related example for detecting a lead-related condition is based on the pacing amplitude that is automatically adjusted by an autocapture system. If the pacing amplitude required to capture exceeds a threshold, the device may declare the existence of a lead-related condition.
However, the inventors of the present disclosure have determined that these conventional methods of evaluating the electrical properties of a lead to detect a lead-related condition are affected by the tissue of the organs in which the leads are implanted. Therefore, the present disclosure addresses the need in the art for a system and method for eliminating the measurement variations that are associated with the lead-to-tissue interface.